The invention relates to a drive device for a bicycle driven by an electric motor, to a bicycle driven by an electric motor comprising such a drive device, and to a bicycle frame comprising such a drive device.
Nowadays, bicycles are frequently used as means of transport and movement. There are different types of bicycles. Full-suspension bicycles are known, for example. These are also referred to in short as fully bikes.
Full-suspension bikes comprise a main frame where a rear linkage element is swingingly supported. A damper element, frequently a steel spring or air damper is provided between the rear linkage element and the main frame.
When designing full-suspension bicycles, there is a conflict of objective between the so-called rocking of the rear linkage element and the so-called pedal kickback. This effect is dependent on the compression and decompression behavior of the rear linkage element relative to the main frame in certain riding dynamics situations.
In particular, riding dynamics situations are considered where the rider applies a high torque using the drive, i.e. using the pedal crank. High torques are particularly applied when starting, riding uphill or accelerating. The force applied when pedaling is guided from the pedal crank to the rear wheel by means of the bike chain.
The rear linkage element tends to compress due to the dynamic bike load distribution occurring when starting, riding uphill, accelerating etc. This compression behavior of the rear linkage element, however, is not desired since a high portion of the drive energy is lost by this.
This is why nowadays the rear linkage element is to be designed such that it has a tendency to decompress when starting, accelerating etc. This decompression behavior can be influenced in dependence on the arrangement of the swing arm bearing relative to the main frame and in dependence on the chain pull direction.
However, too strong of a decompression behavior results in the chain being pulled back, counter-acting the crank movement, due to the decompressing rear linkage element. The result of this is the so-called pedal kickback, which means that, while the rider forces the pedal in one direction, the rear linkage element pulls the pedal in the opposite direction.
This conflict of objective between compression and pedal kickback can be more pronounced in bicycles driven by electric motors, i.e. in so-called e-bikes. The electric motor used in the e-bike may, depending on the set-up of the respective e-bike, be used as the sole driving means, or also as a supportive driving means supporting the pedal movement by the rider. Thus, the electric motor may generate a high torque exceeding the torque provideable by the rider by means of the pedal crank.
Correspondingly, strong a compression and/or an increased pedal kickback usually occur in modern e-bikes.
For illustrating the effects just described, FIG. 13 shows a conventional full-suspension mountain bike 1000. The mountain bike 1000 comprises a main frame 1010 and a rear linkage element 1020. The main frame 1010 comprises a top tube 1011, a saddle tube 1012, a down tube 1013 and a head tube 1014.
The rear linkage element 1020 comprises a seat stay 1021 and a chain stay 1022. The rear linkage element 1020 is connected to the main frame 1010 in a hinged manner. The chain stay 1022 is connected to a swing arm bearing 1023 arranged at the main frame 1010. The seat stay 1021 is connected to a damper 1090 in a hinged manner. In the, in the riding direction, back part, the rear linkage element 1020 comprises a drop-out end 1024 which receives the axis 1050 of the rear wheel 1051.
A pinion packet 1060, which is also referred to as cassette or cogset, is arranged at the wheel hub of the rear wheel 1051. The pinion 1060 comprises several pinions of varying diameters.
The main frame 1010 additionally comprises a bottom bracket 1040. An axis or shaft of a pedal crank 1041 extends through the bottom bracket 1040. The pedal crank 1041 comprises a large chain ring 1042 and a small chain ring 1043.
A drive chain 1030 extends between the front chain rings 1042, 1043 and the rear pinions of the pinion packet 1060. The chain 1030 comprises a top side or tight or load side 1031 of the chain, and a bottom side or slack side 1032 of the chain.
When rotating the pedal crank 1041 (in FIG. 13 in a counter-clockwise direction), the chain 1030 is tensed at the tight side 1031 and transfers the force to the pinion of the pinion packet 1060 engaging in the chain 1030. This force is also referred to as chain pull force FC and extends along a direction of extension of the tensed chain 1030 or tight side 1031. The direction of extension of the tight side 1031 consequently defines the force action line 1070 of the chain pull force FC.
Since the rear wheel 1051 is suspended on the axis 1050 to be rotatable, the result is an axis force FA acting on the axis 1050, the force action line 1080 of which is in parallel to the force action line 1070 of the chain pull force FC.
FIG. 14 shows the same conventional mountain bike 1000 again. The full-suspension mountain bike 1000 comprises a damper 1090 arranged between the rear linkage element 1020 and the main frame 2. FIG. 14 shows the mountain bike 1000 in an unloaded state with no rider. As soon as the mountain bike 1000 is loaded, for example, by the weight of the rider, it will be compressed. The rear wheel 1051 here swings upwards along the wheel trajectory 1052. In so-called single pivots, the wheel trajectory 1052 results from the distance between the swing arm bearing 1023 and the rear axis 1050.
Generally, the rear linkage element 1020 of the mountain bike 1000 is able to both compress and decompress. When compressing, the rear wheel 1051 swings upwards along the wheel trajectory 1052. When decompressing, the rear wheel 1051 swings downwards along the wheel trajectory 1052.
In conventional mountain bikes 1000, the damper 1090 is usually adjusted such that, with static loading by the rider's weight of a rider sitting or standing inactively on the mountain bike 1000, it is compressed by about 10% to 30% of the total spring deflection or spring travel. This region is also referred to as negative spring travel or SAG.
As mentioned already, when compressing, the rear wheel 1051 swings upwards along the wheel trajectory 1052. Conventional mountain bikes 1000 nowadays are constructed such that the rear wheel 1051, in SAG, swings to a point such that the rear axis 1050 is located approximately in the construction point 1055 indicated. The result here is an imaginary line 1081 between the deflected rear axis 1055 and the swing arm bearing 1023. In SAG, this line 1081 is in parallel to the road surface 1082.
In the case of compression or decompression of the rear linkage element 1020, the angle position of this line 1081 relative to the road surface 1082 changes. The angle forming between this line 1081 and the road surface 1082 is referred to as suspension (or spring) oblique (or bias) angle. In SAG, in the conventional mountain bike 1000 shown in FIG. 14, a suspension oblique angle of 0° results.
When starting or accelerating, the rider, by using the pedal crank 1041, generates a torque which is transferred to the rear wheel 1051 via the chain 1030, the cassette 1060 and the hub. In particular in riding situations like starting, accelerating or riding uphill, the rider generates a relatively high torque.
In addition, a rider sitting or standing on the bike 1000 generates a high center of gravity which additionally is shifted horizontally in the direction of the rear wheel 1051. When starting, accelerating or riding uphill, the inertia of the entire system of rider and bike 1000 causes a dynamic wheel load distribution, i.e. the center of gravity migrates further backwards, i.e. in the direction towards the rear wheel 1051. With a non-tuned bike, this dynamic wheel load distribution causes the rear linkage element 1020 to compress. This behavior is also referred to as starting torque pitch or squat.
As mentioned already, the state of the art approach is supporting the starting torque pitch by means of chain pull. The chain pull here provides for decompression of the rear linkage element 1020. What is tried here by means of construction techniques is realizing decompression caused by means of the chain pull, to the same extent to which the rear linkage element would otherwise compress due to inertia. This means that one tries to compensate compression of the rear linkage element, caused by the dynamic wheel load distribution, by decompression, of equal magnitude by means of chain pull. This is to be illustrated using the conventional mountain bike 1000 shown in FIG. 14.
As has been mentioned above, the direction of the force FA acting on the axis 1050 depends, among other things, on the chain pull direction along the force action line 1070 (FIG. 13) for the force FC. In a non-loaded state, the rear axis 1050 at first is located in a bottom position as shown in FIG. 14 where the force FA1 acts on the axis 1050 along the force action line 1071. In a compressed state, and particular in SAG, the rear axis 1050 is located in a deflected top position 1055, wherein the force FA2 acts on the rear axis 1050.
The force FA2 acts along the force action line 1072. As can be recognized from FIG. 14, the force action line 1072 is located between the swing arm bearing 1023.
When the rider, starting from the SAG position, is pedaling and generates a drive torque, the force FA2 along the chain pull direction will act on the rear axis 1050 in the construction point 1055. Since the force action line 1072, as has been mentioned before, is located below the swing arm bearing 1023, the force FA2 causes decompression of the rear linkage element 1020. This means that the force FA2 tries to pull the rear axis 1050 to below the swing arm bearing 1023 so that the rear wheel 1051 moves downward along the wheel trajectory 1052. This decompression movement counteracting the compression movement (starting torque pitch) is also referred to as starting torque pitch support or anti-squat.
Advantageously, it is aimed at, in construction, to arrange the swing arm bearing 1023 relative to the force action line 1072 of the force FA2 acting on the axis 1050 such that magnitude and direction of the chain pull force cause decompression, which, when starting and the like, counteracts and advantageously suppresses completely the compression occurring. When the decompression caused by the chain pull completely compensates the starting torque pitch (compression), this is referred to as neutral bike chassis or 100% anti-squat.
The mode of functioning of the chain pull which is an important component in conventional full-suspension mountain bikes for the anti-squat behavior, can be illustrated easily when describing the angular deviation of the chain pull direction in connection with the magnitude of the chain pull force relative to the wheel trajectory normal.
The wheel trajectory normal is a straight which is arranged at an angle of 90° to the direction of compression, i.e., in the case of a single pivot, the normal will intersect the swing arm point of rotation 1023.
The wheel trajectory normal (perpendicular line to the direction of compression) at the same time describes the suspension oblique angle (angle between wheel trajectory normal and road surface).
If the chain pull direction deviates from the trajectory normal, a force of a compressing or decompressing effect is introduced into the rear wheel suspension. The stronger the chain pull force and the greater the angular deviation to the trajectory normal, the stronger the reaction for the rear linkage element. Thus, in a conventional bike, the anti-squat behavior is differing strongly, depending on the gear set, since the chain pull force differs in direction and magnitude for each gear.
A calculation including the drive power of 200 watts provided by a rider is taken as an example here. This corresponds to a crank force of 140N with 60 RPM:
Case 1: Gear combination of 22 teeth in the front and 34 teeth in the back (typical uphill gearing)
                the result here is a chain pull force of 722N and a drive force of 149N, which in turn results in an anti-squat effect of 120% (too much anti-squat or over-compensation)Case 2: Gear combination of 36 teeth in the front and 11 teeth in the back (typical speed gearing)        the result here are a chain pull force of 543N and a drive force of 34N, which in turn results in an anti-squat effect of 57% (too little anti-squat or under compensation).        
Conclusively, the chain pull force varies strongly in dependence on the gear set. At the same time, the static friction force (drive force) introduced into the ground also varies strongly due to the different gear ratios.
When adding both effects, in a conventional mountain bike, the anti-squat effect varies relatively strongly in dependence on the gear set. The bandwidth of the anti-squat effect in a conventional MTB is between 0 and 200%.
Usually, in an uphill gear (pinion combination of, for example, 22 teeth in the front and 34 teeth in the back), there is a strong chain pull the angle of which, in relation to the wheel trajectory normal, is directed downwards. This results in a rear wheel suspension reacting in a decompressing way, i.e. more than 100% anti-squat. In speed gears (pinion combination of, for example, 36 in the front and 11 in the back), there is lower a chain pull, wherein the direction of effect of the chain pull force relative to wheel trajectory normal is directed upwards. This generates undesired compression of the rear linkage element when pedaling.
In a conventional full-suspension bike, due to these correlations, the desirable most precisely 100% anti-squat exists only for a single specific gear combination.
As has been mentioned before referring to FIGS. 13 and 14, what is strived for is constructing the mountain bike 1000 such that the force action line 1072 and the swing arm bearing 1023 are oriented relative to each other such that the chain pull results in a decompression movement, the magnitude of which corresponds to the magnitude of the starting torque pitch (compression).
In order to realize this, conventional mountain bikes 1000 are constructed such that, in SAG, they exhibit a suspension oblique angle of 0°, i.e. the imaginary connective line 1081 between the swing arm bearing 1023 and the rear axis 1050 is in parallel to the road surface 1082. This offers a good starting point for implementing the direction of the force action line 1072 relative to the swing arm bearing 1023 such that high a starting torque pitch support can be achieved.
Providing a suspension oblique angle of 0° additionally is of advantage in that drive influences which are caused by the kinematics of the rear linkage element, can mostly be ignored so that the starting torque pitch support, i.e. the decompression magnitude, may be determined solely by the chain pull (magnitude and direction).
However, as has been mentioned before, there is the conflict of objective between the starting torque pitch support caused by chain pull and the so-called pedal kickback. Due to the decompression caused deliberately by means of the chain pull, the rear wheel 1051 moves downwards along the wheel trajectory 1052. The result is a change in horizontal length between the rear wheel axis 1050 and the bottom bracket. When decompressing, this horizontal distance between rear wheel axis 1050 and bottom bracket 1040 increases. This in turn causes a corresponding increase in the tight side 1031 of the chain 1030. However, since the chain 1030 is relatively rigid, the chain 1030 pulls the crank 1040 backwards, opposite to the actual pedaling direction.
Conventional mountain bikes 1000, in SAG, usually comprise a suspension oblique angle (wheel trajectory normal) of 0°, which means that the wheel trajectory normal is parallel to the road surface. In conventional full-suspension mountain bikes 1000, the anti-squat is generated exclusively by chain pull, i.e. if the chain pull is directed downwards, the result will be an anti-squat effect. If the chain pull is directed upwards, the result will be a squat effect. Thus, the chain pull direction is dependent on the gear combination selected.
A deviation of the chain pull direction from the trajectory normal in turn will generate an undesired effect on the crank. If the chain pull vector in relation to the trajectory normal is directed downwards, the result will be a back rotation of the crank when compressing, i.e. pedal kickback. If the chain pull vector in relation to the trajectory normal is directed upwards, the result will be a forward rotation of the crank when compressing, i.e. a so-called pedal front kick. This means that nowadays there is a conflict of objective between anti-squat and pedal backlash.
FIGS. 13 and 14 relate to so-called single pivots where drop-out end 1024 for receiving the rear wheel 1051 is arranged at one end of the chain stay 1022 and the swing arm bearing 1023 for hinge-support at the main frame 1010 is provided at the opposite end of the chain stay 1022.
Further well-known concepts, as exemplarily shown in FIG. 15, describe a multi pivot where an additional hinge 1056 is provided, for example below the drop-out end 1024 in the region of the chain stay 1022. This results in a different wheel trajectory 1052 when compared to the single pivot described before. The wheel trajectory 1052 here does no longer follow a simple circular path around the swing arm bearing 1023 with a radius resulting from the distance between the swing support 1023 and the chain stay bearing 1050. Instead, the wheel trajectory 1052 is determined by means of the instant center, while trying to provide a wheel trajectory which is directed perpendicularly upwards to the best degree possible, with no appreciable curvature.
The instant center M1 is the point of intersection of the two straights or the two so-called links 1083, 1084. The first link 1083 is the straight passing through the additional bearing 1056 and through the swing arm bearing 1023. The second link 1084 is the straight passing through the hinge 1025 between the rear linkage element (seat stay 1021) and rocker 1024 and through the pivot point 1026 of the rocker 1024 at the main frame 1010.
For setting up the straight 1081, a connection is drawn between the rear axis bearing 1050 and the instant center M1. In the multi pivot shown in FIG. 15, the frame is also set up such that the line 1081 is in parallel to the road surface 1082 when the bike is in the SAG. In other words, the multi pivot is set such that a suspension oblique angle of 0° results, which means that the anti-squat is controlled by means of the chain pull.
In conventional bikes, like the mountain bikes 1000 just described, providing a starting torque pitch support by means of the chain pull can be realized easily, since the pedaling force provided by the rider, and thus the drive torque introduced to the rear wheel, can be predicted very well for the engineers.
Bicycles driven or supported by an electric motor nowadays are also constructed such that the starting torque pitch support is realized by means of the chain pull. However, in particular in riding situations where large a drive torque acts, like when starting, accelerating or riding uphill, the additional torque of the electric motor causes the rear linkage element of conventional bikes to decompress excessively. On the one hand, this results in an unfamiliar and partly insecure riding behavior of the bike and, on the other hand, in a strong pedal backlash.
Consequently, when constructing the conventional bikes such that the starting torque pitch support works very well for drive by means of muscle power, when starting the electric motor, the rear linkage element will decompress too strongly. If the rear linkage element of a conventional bike, however, is optimized to a starting torque pitch support for the torque of the electric motor, the rear linkage element would compress too strongly when pedaling. This is why, nowadays, compromises have to be accepted in order to satisfy both situations at least to a certain degree.